siox: make siox_bus_type const

Since commit d492cc25

 ("driver core: device.h: make struct bus_type
a const *"), the driver core can properly handle constant struct
bus_type. Move the siox_bus_type variable to be a constant structure as
well, placing it into read-only memory which can not be modified at
runtime.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: "Ricardo B. Marliere" <ricardo@marliere.net>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Thorsten Scherer <t.scherer@eckelmann.de>
Link: https://lore.kernel.org/r/20240204-bus_cleanup-siox-v2-1-3813a6a55dcc@marliere.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>